R--Fe--B permanent magnets have already been proposed (in Japanese Patent Laid-open No. S59-46008/1984, in gazette, and Japanese Patent Laid-open No. S59-89401/1984, in gazette) which have B and Fe as their main components, using light rare earth elements such as Nd and Pr which are plentiful resources, which contain no high-cost Sm or Co, and which offer new high-performance permanent magnets that greatly exceed the maximum performance of conventional rare earth cobalt magnets.
The magnet alloys noted above have a Curie temperature ranging generally from 300.degree. C. to 370.degree. C. By replacing some of the Fe with Co, however, an R--Fe--B permanent magnet is obtained having a higher Curie temperature (Japanese Patent Laid-open No. S59-64733/1984, Japanese Patent Laid-open No. S59-132104/1984). Also proposed (in Japanese Patent Laid-open No. S60-34005/1985) is a Co-containing R--Fe--B rare earth permanent magnet that exhibits a Curie temperature that is at least as high as the Co-containing R--Fe--B rare earth permanent magnet noted above, and a higher (BH)max, wherein, in order to enhance the temperature characteristics, and especially to improve the iHc, at least one heavy rare earth element such as Dy or Th is contained in some of the R in the Co-containing R--Fe--B rare earth permanent magnet wherein such light rare earth elements as Nd and Pr are primarily used as the rare earth element (R), whereby, while maintaining an extremely high (BH)max of 25 MGOe or greater, iHc is raised higher.
There are problems, however, in that the permanent magnets noted above, which are made from R--Fe--B magnetic anisotropic sintered bodies exhibiting outstanding magnetic properties, have as their main component an active chemical compound composition containing rare earth elements and iron, wherefore, when they are built into a magnetic circuit, due to oxides that are produced on the surface of the magnets, magnetic circuit output decline and variation between magnetic circuits are induced, and peripheral equipment is contaminated by the separation of the oxides from the magnet surfaces.
Thereupon, a permanent magnet has been proposed (in Japanese Patent Publication No. H3-74012/1991) wherein the surface of the magnet body is coated with an anticorrosive metal plating layer, by either an electrolytic or non-electrolytic plating method, in order to improve the anticorrosion performance of the R--Fe--B magnets noted above. With these plating methods, however, the permanent magnet body is a porous sintered body, wherefore, in a pre-plating process, acidic solution or alkaline solution remains in the pores, giving rise to fears of degradation over time and corrosion, and the chemical resistance of the magnet body deteriorates, wherefore the magnet surface is corroded during plating so that adhesion and anticorrosion performance are impaired.
Even when an anticorrosive plating layer is provided, in anticorrosion tests in which samples are exposed to a temperature of 60.degree. C. and relative humidity of 90% for 100 hours, the magnetic characteristics proved to be very unstable, exhibiting 10% or greater degradation from the initial magnetic characteristics.
For this reason, it has been proposed (in Japanese Patent Publication No. H5-15043/1993) that, in order to improve the anticorrosion performance of R--Fe--B permanent magnets, an ion plating method or ion sputtering method or the like be used to coat the surfaces of the magnets noted above with AlN, Al, TiN, or Ti. However, the AlN and TiN coatings have crystalline structures, coefficients of thermal expansion, and ductilities that differ from those of the R--Fe--B magnet bodies, wherefore adhesion is poor and, although the adhesion and anticorrosive properties of the Al and Ti coatings are good, their anti-wear performance is poor.
In order to resolve these problems, it has been proposed (in Japanese Patent Laid-open No. S63-9919/1988, in gazette) that the surface of the R--Fe--B permanent magnet bodies be coated with laminated Ti and TiN films. However, the crystalline structure, coefficient of thermal expansion, and ductility of the Ti and TiN coating films differ, so adhesion is poor, peeling occurs, and anticorrosion performance declines.
For these reasons, the inventors, for outstanding anticorrosive permanent magnets exhibiting outstanding adhesion with the foundation, proposed (in Japanese Patent Laid-open No. H6-349619/1994) an anticorrosive permanent magnet wherein, after forming a Ti coating film having a specific film thickness as the foundation film on the surface of an R--Fe--B permanent magnet body, by a thin film forming method, an N diffusion layer wherein the N concentration increases as the surface is approached is formed in the specific film thickness of the surface of the Ti coating film, by a thin film forming method, while introducing a gas mixture of Ar gas and N.sub.2 gas under specific conditions, after which a TiN coating film of a specific film thickness is coated on, in N.sub.2 gas, by a thin film forming method such as ion plating, and (in Japanese Patent Laid-open No. H7-249509/1995) an anticorrosive permanent magnet having an Al coating film of a specific film thickness as the foundation film.
However, while the anticorrosive permanent magnets noted above exhibited outstanding anticorrosiveness in anticorrosion tests at a temperature of 80.degree. C. and relative humidity of 90%, in severe anticorrosion tests such as salt water spray tests (spray tests with 5% neutral NaCl solution under JIS Z2371 test conditions at 34.degree. C. to 36.degree. C.), the anticorrosive performance was inadequate. Thus magnets are needed which will be resistive to salt water spray and exhibit adequate anticorrosiveness even in salt water spray tests, for use, for example, in undulators exposed to the atmosphere.